The present invention is directed to heterocyclic copolymers, particularly benzoxazole copolymer systems containing pendent hydroxyl groups.
Sol-gel processing of ceramics and glass is an area of intense research interest because of inherent advantages compared to more conventional processing. By starting with well mixed solutions or sols, chemical homogeneity even on the molecular scale can be obtained.
A great variety of metal alkoxides are commercially available. Still others have been synthesized for specific uses. The alkoxides are utilized by first partially hydrolyzing the alkoxide, EQU M(OR).sub.v +wH.sub.2 O.fwdarw.M(OR).sub.v-w (OH).sub.w +wROH
where R represents a lower alkyl group and M represents Si, Ti, Al and the like, as well as mixtures thereof. The partially hydrolyzed species are then allowed to link forming M--O--M bonds by a polymerization or condensation reaction.
The majority of work done on sol-gel of polymerized alkoxides has been done for glasses. Much of this centers around SiO.sub.2 glasses or high SiO.sub.2 glasses. The growing use of optical fibers for transmission of information at high rates has provided an incentive to seek fabrication methods for optical grade SiO.sub.2 glass which is less expensive than vapor phase methods. Further, silicon alkoxides exist which are inexpensive, highly pure and easily polymerized to gels. The most common of these is tetraethylorthosilicate (TEOS), Si(C.sub.2 H.sub.5 O).sub.4, the ethoxide of silicon. When an alcohol (e.g., ethanol) is used as a mutual solvent, TEOS can be mixed with water. This mixture is slow to hydrolyze, but the rate can be increased by additions of acids or bases as catalysts. Acid catalyzed gels form transparent gels which appear to be rather uniform polymers. Base catalyzed gels are not as transparent and are thought to contain SiO.sub.2 clusters which then link together to form a gel.
In the case of TEOS, the mechanism for gel formation is polymerization after partial hydrolysis of Si(C.sub.2 H.sub.5 O).sub.4 to have both ethyl groups and hydroxide groups attached to the Si: EQU Si(C.sub.2 H.sub.5 O).sub.4 +wH.sub.2 O.fwdarw.Si(C.sub.2 H.sub.5 O).sub.4-w (OH).sub.w +wC.sub.2 H.sub.5 OH
Reaction of an OH group on one Si with a C.sub.2 H.sub.5 O group on another releases another alcohol molecule and forms a siloxane bond, Si--O--Si, at all temperatures close to ambient. These siloxane bonds form the basis for the polymerization and thus the gelation.
Such gels contain large amounts of water and alcohol, leaving a low density of SiO.sub.2. Drying such gels results in large shrinkages as liquid-filled pores partially collapse. Since the liquid content of the gel is large and the pores are small, liquid transport is slow and rapid drying leads to large shrinkages near the gel body surfaces. Because the mechanical strength of the gel is low, these non-uniform shrinkages lead to cracking.
Several studies have demonstrated the successful incorporation of various functionalized oligomers into a sol-gel network to produce novel organic/hybrid materials referred to as `ceramers`. Such studies have involved, for example, a sol-gel reaction using tetramethylorthosilicate (TMOS) or TEOS and silanol-terminated poly(dimethylsiloxane). Other systems investigated were hybrids based upon TEOS or TMOS, or related metal alkoxides, reacted with an oligomer of poly(tetramethylene oxide) endcapped with isocyanatopropyltriethoxysilane. Wilkes et al, U.S. Pat. No. 5,316,695, disclose the use of a polymeric catalyst, such as poly(styrenesulfonic acid), in such a system.
Organic/inorganic hybrid materials prepared through sol-gel processing have the potential to possess the desired properties of both organic and inorganic components, such as high tensile modulus, scratch resistance, thermal and dimensional stability from the inorganic network, as well as toughness, flexibility and light weight from the organic portion. A variety of high performance, thermally stable polymeric structures are known, but they are intractable and virtually impossible to process. Aromatic heterocyclic polymers are the most attractive high temperature, high performance polymer systems. Although these polymers have excellent high temperature properties, they exhibit solubility only in high boiling aprotic or acidic solvents.
Accordingly, it is an object of the present invention to provide aromatic heterocyclic polymers having improved solubility properties.
It is another object of the present invention to provide aromatic heterocyclic polymers having functionality for co-reaction with metal alkoxides for preparing ceramers.
Other objects and advantages of the present invention will be apparent to those skilled in the art.